Water for the production of drinking water and for the food and pharmaceutical industries must be essentially free from insecticides, herbicides, chlorinated hydrocarbons, phenols, colorants, aroma substances and flavourings and other substances harmful or disagreeable to man. Governments also frequently demand that harmful organic components are removed from industrial effluents.
Physical adsorption on active charcoal is the technology most widely used for the removal of the above mentioned constituents, although extraction, distillation, membrane separation processes and biological processes can also be used to remove organic components from water. Active charcoal is a broad-spectrum adsorbent which can be used to remove undesirable components (colorants, aroma substances and flavourings and foaming agents) and to recover dissolved components by concentration. Active charcoal is characterized by a high specific surface area of 300-2500 m.sup.2 per gram. There are two forms of active charcoal used as adsorbent in liquid phases: powdered charcoal and granular charcoal. The liquid to be treated is mixed with the charcoal particles, heated if necessary and, after a certain adsorption time, separated off by settling of the solid particles or by filtration. To an increasing extent powdered charcoal is being replaced by granular charcoal in connection with ease of handling, the absence of the need for filtration and the possibility of regenerating granular charcoal. The type of active charcoal chosen is determined by the desired physical and chemical properties. The most important physical parameters are specific surface area, pore size distribution, density, specific adsorption capacity, particle size, mechanical strength and resistance to attrition. The most important chemical properties are ash content, ash composition and pH. The customary procedure for regeneration of active charcoal is to heat the latter to a temperature of about 1000.degree. C. Alternatively, regeneration can take place by steam stripping, with which method steam is passed through a bed of charcoal granules and volatile constituents are removed with the steam, solvent regeneration, with which method use is made of a solvent to extract adsorbed components from the charcoal, and wet air oxidation, with which method organic and inorganic compounds are oxidized with oxygen at a temperature of between 150.degree. C. and 340.degree. C. and a pressure of between 7 and 200 bar.
The use of granular charcoal for the removal of constituents from aqueous streams is associated with a number of drawbacks. Granular charcoal is microporous, virtually all of the extracting surface being located within the porous charcoal particles. The diffusion to an adsorption site within the pores is the determining factor for the rate. This process can take from only ten minutes to many hours. The actual adsorption step can go to completion in much less time, for example a few seconds to a few minutes. The slow reaction kinetics as a consequence of mass transfer limitations constitues a serious disadvantage. Another drawback is the loss of binding capacity as a consequence of clogging of pores. This is because active charcoal has a heterogeneous pore system, specified in more detail by macro-, meso- and micropores. As a result of pores being closed off by large molecules, some of the available adsorption capacity becomes inaccessible, which results in a reduction in the adsorption capacity. Finally, active charcoal adsorption can be employed mainly for apolar components. The effect of adsorption on active charcoal is thus dependent on the polarity of the components. Apolar components are firmly bound, whereas more polar components will adsorb to a much lesser extent.
If the removal process is carried out in a column, one is also confronted with the drawbacks that the adsorption capacity is poorly utilized as a consequence of the occurrence of preferential flow in a packed bed, that there is a need for pretreatment of the stream in connection with clogging of the column bed and that there is a question of low volumetric capacity as a consequence of low liquid flow rates in the column.
The disadvantage of the poor utilization of the absorption capacity can be avoided by making use of a fluidized bed. Moreover, no pretreatment is necessary in this case.
A significant disadvantage of existing methods for removing organic constituents from a n aqueous stream with the aid of granular charcoal is that there are no suitable techniques for regeneration of the granular charcoal in situ, i.e. on-line with the adsorption step.
A method of the type indicated in the preamble of the description is disclosed in DE-A 2 743 683. The disclosed method relates specifically to the treatment of effluent originating from the ammoxidation process for the preparation of acrylonitrile or an oxidized liquid obtained by wet oxidation of this effluent. Adsorption is carried out in three mixer-settlers connected in counter-current with long adsorption times (3 times 15 minutes). The method used for regeneration is exclusively wet air oxidation at pressures of between 15 and 150 bar and temperatures of between 150 and 300.degree. C. The oxidation is very intensive and can lead to damage to the particles. As a matter of fact the adsorbent (at least 98% of the adsorbent) has a size lower than 0.04 .mu.m.